This invention relates to a method and a device for measuring the web tension of paper, foils or the like.
At the making or treatment of paper, foils or the like, the material in the form of a web is drawn between conducting rollers, which deflect the web and cause it to be suspended. In this connection it is of great importance, that the tension in the web is maintained within definite limits in order to prevent web break. It is also essential to maintain the web tension at a uniform level across the web, in order to avoid the formation of wrinkles or other unfavourable effects. It is, therefore, desired to be able to measure the web tension continuously in different places along the web and also in several places in the transverse direction of the web. By measuring the web tension continuously, it can be adjusted so that the aforesaid inconveniences are prevented.
The web tension often is measured by utilizing a resonance phenomenon, which arises when the paper web is caused to oscillate transversely between two rollers. In this connection use is made of the fact that the resonance frequency of a material web has a definite relation to the tension in the material.
Devices are also known, at which an intermittent oscillation is applied to a portion of the web, and the transit time for the oscillation to reach another point on the web is indicated. This transit time has a special relation to the web tension.
This latter method involves the disadvantage of being sensitive to external interferences, such as noise. Since the method is to be used in an extremely noisy environment, noise shielding must be provided at the place of measurement. Such shielding is both troublesome and expensive.
According to another type of web tension gauge, which is disclosed in the Laid-Open Document Sweden No. 7603778-7, the web is subjected to transverse oscillations between two support places by a periodically varying force with a definite amplitude and frequency. The oscillation amplitude is measured and constitutes a measure of the web tension. The frequency of the force shall be either slightly above or below the expected basic resonance frequency of the web at the possible web tensions, i.e. the web tension, at which resonance occurs, shall be either slightly higher or slightly lower than any expected tension of the web. The measuring range then will be within a range where small variations in web tension yield great changes in amplitude. By measuring the amplitude, the web tension can be obtained by means of a calibration curve. It is, thus, necessary to carry out calibration in order to obtain a frequency yielding a suitable measuring range. If the web tension varies beyond the measuring range, the frequency must be changed and another calibration curve be utilized.
A variant of this web tension gauge, according to SE-PS No. 7802788-5, utilizes the relation between the resonance frequency and the relation between the web tension and the mass per unit area, and the phase shift between the oscillation of the web and the variations in pressure causing the web to oscillate or the signal effecting the pressure variations.
The resonance frequency is proportional to the root of the relation between web tension and mass per unit area. It is, thus, possible to obtain the web tension by a separate measurement of the mass per unit area.
The aforesaid gauges, which are based on the resonance between two supports, are designed for contacting the paper web. This restricts their use and implies high requirements on the application of the gauge. Contact causes friction against the web, and thereby the web tension is affected. Contact further causes wear and deposits of dust on the measuring head.